In recent years, along with an improvement in performance of electronic equipment, there has been a growing demand for a cell having a larger capacity and a longer life. Conventional lithium-ion cells are limited in capacity since they have almost reached their theoretical limits of an energy density per volume and, therefore, can hardly be expected to achieve any further significant improvement in performance. With this being the situation, attention has been focused on a fuel cell that is significantly improved in energy density per volume compared with conventional cells and thus can achieve an increased capacity.
For example, Patent Document 1 proposes a regenerative fuel cell system. FIG. 11 is a schematic view showing a reaction mechanism of a fuel cell during power generation, and FIG. 12 is a schematic view showing a reaction mechanism of the fuel cell during charging. A fuel cell 520 described in Patent Document 1 is composed of a fuel electrode 521, an oxygen electrode 522, and an electrolyte membrane 523. At the time of power generation, upon supply of hydrogen to the fuel electrode 521, protons and electrons are generated from the hydrogen, and the protons move from the fuel electrode 521 through the electrolyte membrane 523 to the oxygen electrode 522. At the oxygen electrode 522, oxide ions generated from oxygen react with the protons that have moved thereto to generate water, and by these electrochemical reactions, motive power is generated. Furthermore, at the time of charging, upon application of voltages of opposite polarities to the fuel electrode 521 and to the oxygen electrode 522, respectively, reactions reverse to the reactions that occur during power generation occur at the fuel electrode 521 and at the oxygen electrode 522, respectively, so that hydrogen is generated at the fuel electrode 521, and oxygen is generated at the oxygen electrode 522. The fuel cell system described in Patent Document 1 performs charging by taking the hydrogen generated at the fuel electrode 521 into a hydrogen occlusion material and power generation by emitting the hydrogen thus taken thereinto.